The present invention relates to an optical measuring device to optically measure microdisplacement and/or surface roughness of a substance to be measured.
It is well known to utilize a laser ray as a light source of such an optical measuring device. In such devices, microdisplacement and/or surface roughness of the substance to be measured can be measured by utilizing the off-focus of an objective lens, e.g. as disclosed in Japanese Laid Open Patent Application No. 7246/1981.
FIGS. 1 and 2 show an above-mentioned type conventional optical measuring device. Referring to the drawings, 1 designates a laser ray source, 2 is a collimator lens and 3 is a polarizing prism. The polarizing lens 3 is placed such that the incident ray or beam from the laser light source is transformed to a polarized ray or beam. Reference numeral 4 designates a quarter wavelength plate, 5 is an objective lens, and 6 is a reflective substance to be measured. A prism 7 guides a reflected ray or beam from the polarizing prism 3 to a divided light receiving photo-sensor 8 which will be described later. The reflecting surface 7c of the prism 7 is set to be about the critical angle relative to the optical axis of the reflected ray. The divided light receiving photo-sensor 8 has receiving surfaces 8A and 8B which are formed by dividing the surface 8 and the receiving surfaces are substantially normal to the incident surface of the prism 7. The division line 8C crosses the optical axis C of the reflected light at one point.
In operation of the device shown in FIGS. 1 and 2, a laser ray from the laser ray or beam source 1 is transformed into a parallel ray by the collimator lens 2 and passes through the polarization prism 3 as polarized ray P. The polarized ray P from the prism 3 passes the quarter wavelength plate 4 and is transformed into a circuler polarized ray. Then the laser ray is condensed by the objective lens 5 on the surface of the reflecting substance 6.
The laser ray impinging on the reflecting substance is reflected therefrom and passes through the objective lens and the quarter wavelength plate 4. The ray passing through the plate 4 is polarized by the polarization prism 3 which reflects the ray to the prism 7. The ray reflected from the reflecting surface 7C of the prism 7 impinges on the divided photo-sensor 8 which detects the quantity of the ray.
When the surface of the reflecting substance 6 which is to be measured coincides with the focus of the objective lens 5, the reflected light through the objective lens 5 is a parallel ray so that the incident angle of the reflected ray to the prism 7 is the same all along the surface. Thus, the quantity of the reflected ray received by the photo-sensor 8 is uniformly distributed. When the surface of the reflecting substance 6 is displaced from the focus of the objective lens 5 in the direction of arrow X shown in FIG. 1, the reflected ray is a divergent ray after passing through the objective lens 5, so that an incident reflected ray located above the optical axis has a smaller incident angle compared to the parallel ray thereby resulting in a lower reflection factor. On the other hand, a reflected ray located below the optical axis has a higher incident angle compared to the parallel ray thereby resulting in a higher reflection factor. Consequently, the quantity of the reflected ray received by the photo-sensor 8 is not uniform and the quantity of the received ray on the light receiving surface 8A increases.
When the surface of the reflecting substance 6 displaces in a direction opposite to the direction X shown in FIG. 1, the reflected ray is a convergent ray after passing through the objective lens 5, so that the relation of the incident angle of the reflected ray is opposite to the above-mentioned divergent ray. Thus, the quantity of the reflected ray received on the light receiving surface 8B increases.
Thus, conventionally, by obtaining the output difference between the light receiving surfaces 8A and 8B of the divided photo-sensor 8, the relative positions between the surface portions of the reflecting substance 6 and the focus of the objective lens 5 are detected so that microdisplacement and/or surface roughness of the substance to be measured can be obtained.
However, in such a conventional measuring device, fluctuations of the quantity of the ray and non-uniformity of the quantity distribution caused by noise which essentially entrains in the laser ray source 1 are also detected by the photo-sensor 8. Thus, quantity change corresponding to the position of the reflecting substance 6 is disturbed. Both changes can not be distinguished from each other by such a conventional device.